It seems like feGaussianBlur does not blur pixels at the edge of the filter region correctly. It should act as if there are pixels outside the filter region, and as if those pixels are transparent black.

Created attachment 731558[details]
This is how it looks on my machine
This is how it looks on my machine with my changes. However, I am unable to determine if this is how it ought to look. It didn't render properly in inkscape. I then installed and checked against google-chrome and it looks slightly different (the blur seems more pronounced on chrome - the lines are not so clear).

(In reply to O S K Chaitanya from comment #4)
> Created attachment 731558[details]
> This is how it looks on my machine
>
> This is how it looks on my machine with my changes. However, I am unable to
> determine if this is how it ought to look. It didn't render properly in
> inkscape. I then installed and checked against google-chrome and it looks
> slightly different (the blur seems more pronounced on chrome - the lines are
> not so clear).
The blurring on Chrome might be a bit bogus. We had reports about dithering on blurred content. Your result seems to look correct.

(In reply to Robert Longson from comment #7)
> If you look at
> http://www.w3.org/Graphics/SVG/Test/20110816/harness/htmlObjectApproved/
> filters-overview-01-b.html you should see that the fillPaint and strokePaint
> blue boxes are not blurred at the edges. I think that's because of this
> issue.
They are blurred in the PNG. But this is a different issue IMO. Pseudo primitives can be of infinite size. And we still did not clarify if subregions clip the input or the output of a filter primitive (or maybe both).
For input clipping, the edges should be blurred, for output clipping they may shouldn't since fillPaint/strokePaint are of infinite size.

Created attachment 731567[details]
render of w3g's filters-overview-01-b.svg test with my changes
This is how the link above looks with my changes in the Nightly. If I assume that the difference in colour is to be expected, I still find that the nature of blur looks different in image on the left (svg) and on the right (png). Is that fine?

Created attachment 731677[details]
filt3.svg rendered by removing the scaledDivisor optimisation from nightly
I have added renders from Chromium-browser and from Opera for comparison and an attachment with a render from the nightly with my changes.
The current attachment is a render from nightly after applying my changes and then removing the 'scaledDivisor' optimisation (i.e just dividing by boxSize instead of using 'scaledDivisor').
If you look at the renders you will see that this one is quite similar to the renders from Opera and Chromium.
On the other hand, the render with the 'scaledDivisor' optimisation has strong edges visible near the innermost part of the blurred region. Also at the corners you will see that the whitish squares are more prominently visible in the version with the 'scaledDivisor' optimisation.
Given this observation, what do you folks think we should do?
(I tried to get a render from Squiggle but some bug causes it to not render when I zoom in.)

Created attachment 731681[details]
the w3g test rendered with nightly without 'scaledDivisor' optimisation
If you see the w3g test render from the nightly without 'scaledDivisor' opsimisation, you can see that the blur on the edges looks similar to the reference in this image.
In the earlier case (with the 'scaledDivisor' optimisation), it had a lot more 'white' in the edges.

You could post both patches but I imagine that going without an optimisation with visible effects is best as a first cut. We could always add in the optimisation later and then back it out if anyone complains.

Created attachment 732379[details][diff][review]
fix for this bug (after removing the 'scaledDivisor' optimisation)
I have implemented a fix for this bug and have also removed the 'scaledDivisor' optimisation since it causes easily perceptible difference in the resulting render.

Comment on attachment 732379[details][diff][review]
fix for this bug (after removing the 'scaledDivisor' optimisation)
You'll need a review from an SVG Peer: https://wiki.mozilla.org/Modules/Core#SVG
roc (Robert O'Callahan) wrote the ComputeScaledDivisor optimisation. Jonathan Watt created the bug and I've shown some interest here so we're all reasonable choices for this. Let's see if Jonathan wants this?
Do you have tests? If so could you attach them to the patch? If not we really ought to have some.

Ahh, sorry about that. My Bad!
I just looked at the log for this file and the last 3 revisions were by dzbarsky. So I marked him on the r? flag. From now, I'll ensure I am marking it to someone who is an SVG Peer.
Also on an unrelated note, if I wanted to bring a bug to someone's attention, what should I do? For instance, I wanted to have someone look at the comment I posted on bug#522866 but I don't know how to draw someone's attention to it.

Comment on attachment 732379[details][diff][review]
fix for this bug (after removing the 'scaledDivisor' optimisation)
I found some issues while making test cases for this filter. Please ignore this patch. I'll commit another one after fixing the problems.

I didn't realise when I made comment 17 that we already had the "scaledDivisor" optimisation. We shouldn't remove that in this patch, it should just do the region change. We could then consider removing the optimisation in another bug as they are different things.
Peers and module owners generally look at changes to bugs in their area whether or not they are subscribed. Just commenting brings things to our attention. If that doesn't work you could always tick the Need more information checkbox below, choose other and then add one of us.